printk: Add is_printk_legacy_deferred()

If printk has been explicitly deferred or is called from NMI
context, legacy console printing must be deferred to an irq_work
context. Introduce a helper function is_printk_legacy_deferred()
for a CPU to query if it must defer legacy console printing.

In follow-up commits this helper will be needed at other call
sites as well.

Signed-off-by: John Ogness <john.ogness@linutronix.de>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/20240820063001.36405-24-john.ogness@linutronix.de
Signed-off-by: Petr Mladek <pmladek@suse.com>

diff --git a/kernel/printk/internal.h b/kernel/printk/internal.h
index 44468f3828f3ef6d3b48d48f2dc2369bc9533bb6..84706c1c934b25355805e62d0865713f3eb071f7 100644 (file)
--- a/kernel/printk/internal.h
+++ b/kernel/printk/internal.h
@@ -75,6 +75,7 @@ bool printk_percpu_data_ready(void);
        } while (0)
 
 void defer_console_output(void);
+bool is_printk_legacy_deferred(void);
 
 u16 printk_parse_prefix(const char *text, int *level,
                        enum printk_info_flags *flags);
@@ -138,6 +139,7 @@ static inline bool console_is_usable(struct console *con, short flags)
 #define printk_safe_exit_irqrestore(flags) local_irq_restore(flags)
 
 static inline bool printk_percpu_data_ready(void) { return false; }
+static inline bool is_printk_legacy_deferred(void) { return false; }
 static inline u64 nbcon_seq_read(struct console *con) { return 0; }
 static inline void nbcon_seq_force(struct console *con, u64 seq) { }
 static inline bool nbcon_alloc(struct console *con) { return false; }

diff --git a/kernel/printk/printk_safe.c b/kernel/printk/printk_safe.c
index 4421ccac311336f4e45b4149d5dbafa75ee7d70e..86439fd20aabf72bd483ec36644a9f662d4fc08b 100644 (file)
--- a/kernel/printk/printk_safe.c
+++ b/kernel/printk/printk_safe.c
@@ -38,6 +38,15 @@ void __printk_deferred_exit(void)
        __printk_safe_exit();
 }
 
+bool is_printk_legacy_deferred(void)
+{
+       /*
+        * The per-CPU variable @printk_context can be read safely in any
+        * context. CPU migration is always disabled when set.
+        */
+       return (this_cpu_read(printk_context) || in_nmi());
+}
+
 asmlinkage int vprintk(const char *fmt, va_list args)
 {
 #ifdef CONFIG_KGDB_KDB
@@ -50,7 +59,7 @@ asmlinkage int vprintk(const char *fmt, va_list args)
         * Use the main logbuf even in NMI. But avoid calling console
         * drivers that might have their own locks.
         */
-       if (this_cpu_read(printk_context) || in_nmi())
+       if (is_printk_legacy_deferred())
                return vprintk_deferred(fmt, args);
 
        /* No obstacles. */